[120] _On a change produced by Exposure to the Beams of the Sun, in the properties of an elementary substance_, by Professor Draper; _On the changes which bodies undergo in the dark_, by Robert Hunt: Report of the Thirteenth Meeting of the British Association, vol. xii,--_Description of the Tithonometer, an instrument for measuring the chemical force of the Indigo-tithonic rays_: by J. W. Draper, M.D.--Philosophical Magazine, Dec. 1843, vol. xxiii. [121] For several illustrations of this remarkable phenomenon, see _On the Action of the Rays of the Solar Spectrum on Vegetable Colours, and on some new Photographic Processes_; by Sir John F. W. Herschel, Bart., K.H., F.R.S.--Phil. Trans. June, 1842, vol. cxxxiii.; _On certain improvements on Photographic Processes described in a former communication, and on the Parathermic Rays of the Solar Spectrum_; by Sir John F. W. Herschel, Bart., K.H., F.R.S., &c., in a letter addressed to S. Hunter Christie.--Phil. Trans. 1843, vol. cxxxiv. [122] Sir J. F. W. Herschel; see also _Researches on Light_, by the Author. [123] Attention has been directed to the protecting action of certain rays of the spectrum by Sir John Herschel and others. See the Eighteenth Report of the British Association for an experiment by the Author, in which it was proved that all the LIGHT rays protected photographic papers from chemical change, and, therefore, convincingly show that light and actinism were not similar powers. [124] “Having noticed, one densely foggy day, that the disc of the sun was of a deep red colour, I directed my apparatus towards it. After ten seconds of exposure, I put the prepared plate in the mercury box, and I obtained a round image perfectly black;--the sun had produced no photogenic effect. In another experiment, I left the plate operating for twenty minutes; the sun had passed over a certain space of the plate, and there resulted an image seven or eight times the sun’s diameter in length; it was black throughout, so that it was evident, wherever the red disc of the sun had passed, not only was there a want of photogenic action, but the red rays had destroyed the effect produced previous to the sun’s passage. I repeated these experiments during several days successively, operating with a sun of different tints of red and yellow. These different tints produced nearly the same effect; wherever the sun had passed, there existed a black band.”--Mr. Claudet, _On different properties of Solar Radiation, modified by coloured glass media, &c._: Phil. Trans. 1847. Part 2.